Why do leaves change colour?

It’s Calan Gaeaf in Wales today, the first day of winter. Over the past couple of weeks there’s been a change in the weather. Now, when the autumn mists clear I can see swathes of copper in the forested hillsides opposite my window. The chemistry blog Compound Interest has a post explaining the reactions I can see happening in front of me.

The season has also brought an interesting story to the New York Times, Why Does Fall Foliage Turn So Red and Fiery? It Depends. I think it’s good at exploring how leaf colours change in autumn, but it also highlights some of the mystery that we still aren’t sure about why.

It caught my eye as last week I got a Google alert about a chapter from the book Defensive (anti-herbivory) Coloration in Land Plants, called Spring Versus Autumn or Young Versus Old Leaf Colors: Evidence for Different Selective Agents and Evolution in Various Species
and Floras by Simcha Lev-Yadun. He puts forward a few reasons why leaf colours change.

One is obviously that it’s a physiological reaction. The tree is taking back what it needs and that just leads to a change in colour. Another explanation is that it’s to aid fruit display. The changing leafs signal that fruits are ready for dispersal. This wouldn’t be a conscious decision, simply that the plants that most effectively display their fruiting would disperse more effectively and out compete other plants. Finally, it could be related to defences against herbivores – and connected in that way to physiology.

If there are advantages to leaf colour that aren’t seasonal, then the obvious question is why don’t some plants use colours like red at other times of the year. But a quick visit to the garden centre shows you some do. So why green or red?

Lev-Yadun lists some of the ideas for why some plants have young red leaves. The first he mentions is that young red leaves look like older dead ones, so they’re protected from herbivory. This is something that I can see makes sense – until he gives another reason other researchers have proposed.

Young red leaves could attract herbivores and and keep them away from the older green leaves that a plant has put so much effort into building. Again, in isolation it makes sense, but it’s also 180° from the previous explanation.

However, while predation is a problem year-round, Lev-Yadun notes that not all predation is the same. He cites a recent paper in Annals of Botany by Caldwell et al. that points out leaves face different dangers as they grow. A young leaf will be chomped, physically destroying it, but an older leaf may be attacked by aphids, who suck out nutrients instead of eating the leaf whole. This means a leaf has differing defence priorities as it grows, leading to physiological differences and maybe different colouring.

If you’re looking for one definitive answer, then you’ll be disappointed to learn that leaf colour isn’t solved yet. However, if you’re looking for a research project then Lev-Yadun’s chapter is an excellent place to start for an overview and to get an idea of the scientific context for potential research. Unfortunately the cost means this is a book to get from a library.

(and in a case of the pot calling the kettle black, the Annals paper isn’t cheap either – but it will be free access from February 2017)